The purpose of the proposed research is to investigate, at a biochemical and cytological level, the structural organization of human chromosomes and to determine the interrelationship of chromosome structure, the control of chromosome replication and X-inactivation, and sister chromatid exchange (SCE) formation. Many of the biochemical experiments will explore aspects of chromosome replication kinetics and SCE formation characterized in cytological material using BrdU-dye techniques. Emphasis will be placed on an analysis of the chemistry of the SCE process and the significance of SCEs in DNA repair, the response of cells to mutagen-carcinogens, and the increased susceptibility to neoplasia in recessively inherited human chromosome fragility diseases. Biochemical and cell biological techniques, some of which involve fluorescent activated cell and chromosome sorting, will be developed and coordinated to study chromosome structure and function.